1. Field of the Invention
The present invention relates to a power-on reset circuit, and in particular to a power-on reset circuit for outputting a predetermined reset signal.
2. Description of the Related Art
When a power source is initially activated, a voltage generated from the power source can not immediately reach a predetermined voltage value. If the voltage which does not reach the predetermined voltage value is applied to a digital circuit, it may cause the initial situation of the digital circuit to be unknown. Generally, the power-on reset circuit is integrated in Very Large Scale Integration (VLSI) circuits for generating a reset signal. Therefore, when the power source is initially activated, the internal circuit of VLSI circuit can obtain a known initial situation according to the reset signal.
FIG. 1 is a diagram illustrating the power-on reset circuit applied to a circuit device. In FIG. 1, when a circuit device 150 is supplied with electric power, the voltage of a Power node VDD coupled to a power-on reset circuit 100 increases from a ground voltage to a predetermined voltage value, such that the power node VDD charges a capacitor CP through a resistor P1 composed of a PMOS transistor. A control circuit 130 generates a reset signal to the circuit device 150 according to the voltage of the capacitor CP. When the electric power is removed from the power node VDD, the capacitor CP discharges its power to the power node VDD (at this time, the voltage of the power node VDD is close to the ground voltage) through the resistor P1. However, because the resistor P1 is composed of PMOS transistor, the voltage of the capacitor CP can only be dropped to a threshold voltage of the P-N junction in a short time. That is to say, the capacitor CP can not be discharged completely. The voltage of the capacitor CP can not be dropped to a voltage value equal to the reference ground GND. Therefore, when the circuit device 150 (power-on reset circuit 100) is supplied with electric power the next time, the voltage that is preexisted in the capacitor CP drives the control circuit 130 to generate an incorrect control signal, which would further incapacitate the power-on reset circuit 100.
Therefore, a new power-on reset circuit is needed. During the discharging operation, the voltage of the capacitor of the power-on reset circuit is able to be dropped to a voltage value equal to the ground voltage. Thus, the next time the power-on reset circuit is supplied with electric power, the control circuit of the power-on reset circuit can generate the correct control signal, or enable the power-on reset circuit to work normally.